Recordings of sonar signals are used to train sonar operators. Magnetic recordings of an acoustic target provide training realism flexibility at low cost. As a target moves toward or away from the sonar receiver, the signals should be shifted in frequency in proportion to the speed of separation. It would be desirable if an instructor could Doppler shift recorded signals sent to the student to simulate motion of an acoustic target.
Doppler occurs when the range is increasing or decreasing between the sonar sensors and the target. The amount of frequency shift due to Doppler (D) is a function of range rate (r) and the velocity of sound (c) in the ocean. EQU D=(c-r)/c (1) EQU f.sub.o =f.sub.s D (2)
Where
f.sub.o = observed frequency PA1 f.sub.s = source frequency
Representative Doppler ratios for +/-60 Knots of range rate are 0.98 to 1.02. For closing range rates, the frequency of the arriving signal increases.
U.S. Pat. No. 2,809,445 to Hartig et al describes a mechanical training device using synthetic signals simulated by a crude generator. U.S. Pat. No. 2,863,227 to Chubb et al discloses recording of video and audio information for a sonar operator during an actual run while tracking a submarine for use in training.
U.S. Pat. No. 3,040,445 to McClelland discloses a device for sonar training including means for automatically making magnetic tape recordings for correlating the echo-ranging signals of sonar and radar sets with training display equipment.
U.S. Pat. No. 3,713,081 to Murphree discloses a sonar trainer including Doppler effects generator wherein Doppler simulation is achieved by converting the transmitted sonar signal to digital form, reading it into a memory means at a predetermined rate, reading it at a different rate determined by rate of change of range and reconverting the read-out to analog form for processing by an echo generator.
U.S. Pat. No. 3,829,596 Murphree discloses a sonar reverberation simulator involving a digitizer for an orginal sonar signal, storing same in memory means, each signal at a different address, read-out means, digital to analog converter and means for adjusting the amplitude for reverberation simulation.
U.S. Pat. No. 3,835,234 to Murphree discloses a synthesizer for submarine echoes wherein the strength of the echo points can be made a function of aspect or nature of the object, including digitizing, storage, read-out and a multiplying digital-to-analog convertor.